GEAR PUMP
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A Project Repor Reportt on
DESIGN AND FABRICATION OF GEAR PUMP Course Code: M E 251 2 51 Course Title:
M anufa anufacturi cturing ng P rocess rocesses es
Submitted By:
Submitted to:
Muhammad Saqib Anwar
Engr Akhtar Khurshid
Muhammad Nouman
Assistant Professor
Mudassir Hussain
Department of Mechanical
Hamza Ahmed
Engineering, College of
32-Mechanical (B)
E&ME, NUST
Date: May 29 th, 2012
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Table of Contents A bs tr ac t ........................................ ........................................................ ................................. ................................. .................................. .................................. ................... ... 1 Project Overview .............................................................................................................. 1 External Gear Pump ........................................................................................................ 1 Theory Theory of Operation Operation ............................................ ............................................................. .................................. ................................. ............................ ............ 2 Literature Review ............................................................................................................. 3 Te c hn ic al D e tai ls ................................. ................................................. ................................. .................................. .................................. ............................ ........... 4 Gears ................................. .................................................. ................................. ................................. .................................. .................................. ......................... ........ 4 Bearings Bearings ............................... ................................................ .................................. .................................. .................................. ................................. ...................... ...... 6 Shafts Shafts ................................. .................................................. ................................. ................................. .................................. .................................. ......................... ........ 7 Housing Housing ................................. ................................................. ................................. .................................. .................................. ................................. ...................... ...... 8 M an ufa ctu ring ri ng P ro ce sses ss es In vo lve d ........ ............. .......... .......... ......... ......... .......... ......... ......... .......... ......... ......... .......... ......... ......... .......... ....... 9 Gear Hobbing Hobbing............................. .............................................. ................................. ................................. .................................. ................................. ................10 Sand Casting Casting................................. .................................................. ................................. ................................. .................................. .............................. .............11 Machining Machining ............................... ................................................ .................................. ................................. .................................. .................................. .................. .. 11 Grinding.... Grinding.................... ................................. .................................. .................................. ................................. ................................. ................................. ................12 Press Fitting Fitting ................................. .................................................. ................................. ................................. .................................. .............................. .............13 Ca lcu latio ns for D isc har ge R ate of a Ge ar Pu m p ...... ......... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ...... ..... .. 14 A dvan tages and A pplicatio ns of a n Exte rnal Ge ar Pu mp .... ...... .... .... .... .... .... .... .... .... .... .... .... .... .... .... .... .... 15 C on cl us io n ............................. ............................................... .................................. ................................. .................................. .................................. ....................... ...... 17 R e fe re nc es ............................................ ............................................................ ................................. .................................. .................................. .......................... ......... 17
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DESIGN AND FABRICATION OF GEAR PUMP
Abstract Gear pump is a mechanical device which is used in numerous industrial applications. It uses the meshing of gears to pump fluid by displacement. Since the gear pump requires high dimensional accuracy and precise machining, one has to be very adept while manufacturing it. The main impetus for this project is to get familiarized with various fabrication techniques and to enhance our manufacturing skills.
Pr oject Overview A gear pump uses the meshing of gears to pump fluid by displacement. They are one of the most common types of pumps for hydraulic fluid power applications. Gear pumps are also widely used in chemical installations to pump fluid with a certain viscosity. There are two main variations; external gear pumps which use two external spur gears, and internal gear pumps which use an external and an internal spur gear. Gear pumps are positive displacement (or fixed displacement), meaning they pump a constant amount of fluid for each revolution. The motive of this project is to design and fabricate an external gear pump.
Externall Gear Pum ps Externa External gear pumps are a popular pumping principle and are often used as lubrication pumps in machine tools, in fluid power transfer units, and as oil pumps in engines. External gear pumps can come in single or double (two sets of gears) pump configurations with spur spur (shown), helical, and herringbone gears. Helical and herringbone gears typically offer a smoother flow than spur gears, although all gear types are relatively smooth. Large-capacity external gear pumps typically use helical or herringbone gears. Small external gear pumps usually operate at 1750 or 3450 rpm and larger models operate at speeds up to 640 rpm. External gear pumps have close tolerances and shaft support on both sides of the gears. This allows them to run to pressures beyond 3,000 psi / 200 bar, making them well suited for use in hydraulics. With four bearings in the liquid and tight tolerances, they are not well suited to handling abrasive or extreme high temperature applications. Tighter internal clearances provide for a more reliable measure of liquid passing through a pump and for greater flow control. Because of this, external gear pumps 1
are popular for precise transfer and metering applications involving polymers, fuels, and chemical additives.
Fig ur e 1. Exploded Exploded View View of an External External Gear Gear Pump
Theory of Operation External gear pumps are similar in pumping action to internal gear pumps in that two gears come into and out of mesh to produce flow. However, the external gear gear pump uses two identical gears rotating against each other -- one gear is driven by a motor and it in turn drives the other gear. Each gear is supported by a shaft with bearings on both sides of the gear. 1 . As the gears come come out of mesh, they create expanding volume on the inlet side of the pump. Liquid flows into the cavity and is trapped by the gear teeth as they rotate. 2 . Liquid travels around around the interior of the casing in the pockets between the teeth and the casing -- it does not pass between the gears. 3 . Finally, the meshing of the gears gears forces liquid through the outlet outlet port under pressure. 2
Because the gears are supported on both sides, external gear pumps are quietrunning and are routinely used for high-pressure applications such as hydraulic applications. With no overhung bearing loads, the rotor shaft can't deflect and cause premature wear.
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2
3
Figu Fi gu r e 2. Pumping Pumping Action Action of an External External Gear Gear Pump
Literature Lit erature R eview Gear pumps are among the oldest and most commonly used pumps within the industry. Though the gear pump is extremely simple in its operating principle, the fundamental understanding of the instantaneous pump flow has been a subject of considerable interest for many years. The complexity of this subject arises due to the nature of the geometry involved, which has typically required numerical analysis to solve the governing equations. Much of the recent research that is most germane to gear pump technology is briefly summarized in the following paragraph. Research pertaining to the average flow rate of the gear pump has been conducted by Frith and Scott [1]. In their work, the authors have related the degradation of the average flow rate to the online generation of wear debris. In other research, authors have emphasized a prediction in the fluid film thickness between the gear end-face and the end wear-plate [2]. In this work, the authors were primarily concerned with volumetric leakage and pump efficiency. For predicting the cyclic moments and forces on the pump shaft, Foster, Taylor, and Bidhendi [5] conducted an in depth analysis of the gear pump using a computer program for generating solutions. This work considered the trapped volume of fluid between meshing teeth and the results were shown to compare nicely with actual test data. Still, others have focused on various tooth geometries in an effort to reduce and/or compare the discharge flow amplitude of the pump [7]. Though all of this work has been valuable in and of itself, none of this work has considered a comparison of same-size external gear pumps, which use different numbers of teeth for the driving and driven gears. The question related to tooth number is significant since other positive-displacement pump types 3
tend to exhibit different flow characteristics depending upon the number of discrete pumping elements that are used. For instance, axial piston pumps have been shown to exhibit significantly different pulse shapes for pumps that use an even versus an odd number of pistons [8]. This present study is aimed at design and fabrication of an external gear pump.
Technical Details From an engineering perspective, Product design is a critical activity because it has been estimated that, generally, 70 to 80% of the cost of product development and manufacture is determined at the initial design stages. So, designing a product meeting the international standards (ANSI, ASME, ASTM, ISO, SAE etc.) is a prerequisite to machine functionality and design viability. Keeping in view the above scenario, we devised a Gear Pump optimized for industrial applications. The specifications of various components of an External Gear Pump are enlisted below.
i. Gears A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear, however a gear can also mesh a non-rotating toothed part, called a rack, thereby producing translation instead of rotation. The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that the teeth of a gear prevent slipping. In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in first gear, refers to a gear ratio rather than an actual physical gear. The term is used to describe similar devices even when gear ratio is continuous rather than discrete, or when the device does not actually contain any gears, as in a continuously variable transmission. The gear used in our project was a spur gear.
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Fig ur e 3. Gear Gear No Nomen mencla clatur turee
Gear Specifications The specifications of gear used in our project are enlisted in the table given in the proceeding page. Below is a diagram representing the nomenclature of a spur gear.
Figure 4. Nomenclature of a Spur Gear
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Material
Diametral Pitch (P) (in.)
N o. of of teeth (N )
Pitch Dia. (d) (in.)
Bore (B ) (in.)
Outside Dia. (D) (in.)
Face ( l) (in.)
5
11
2.4
1.0625
2.8
1.750
Steel NK11B
ii. Bearings A bearing is any of various machine elements that constrain the relative motion between two or more parts to only the desired type of motion. This is typically to allow and promote free rotation around a fixed axis or free linear movement; it may also be to prevent any motion, such as by controlling the vectors of normal forces. Bearings may be classified broadly according to the motions they allow and according to their principle of operation, as well as by the directions of applied loads they can handle.
Figu Fi gu r e 5. Cross Sectional Sectional Views Views of a Ball Bearing Bearing
Bearing Specifications Boundary dime nsions nsions (mm) Part No.
Bore (A)
62 0 5
25
Outer diameter (B) 52
Basic load ratings (N )
Width (C)
Dynamic (Cr)
Static (Cor)
15
140000 1400
7900
6
Weight (lb)
0.810
Bearing Material Specifications Material Used: CS6208XXPKWS C
S
XX
PK
WS
Ball material: C=Ceramic (Si3N4) No symbol = Chrome Steel
Ring Material: No symbol = Chrome steel S=Stainless Steel (440C)
Closure: XX=Open (Standard bearing width, no seal) No Symbol = Open (Narrow bearing width, no seal)
Retainer material: PK=PEEK No Symbol = Metal
Lubrication: Dry film lubrication/ Coating WS=Tungsten Disulfide
iii. Shaft Shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement between them. Shafts are carriers of torque: they are subject to torsion and shear stress, equivalent to the difference between the input torque and the load. They must therefore be strong enough to bear the stress, whilst avoiding too much additional weight as that would in turn increase their inertia. M aterial U sed : Mild Mild St Stee eell
Front View of a shaft connected to
Front View of a shaft driving the gear
the driven gear.
Figu Fi gu re 6. Single view view Orthogra Orthographic phic Projecti Projections ons of Shafts
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iv. Housing Housing is an important component of gear pump, which is used in order to prevent leakage. This factor is quite significant while designing a gear pump. Below are the 2-D sketches of various components used in housing of gear pump. The Material used was aluminum.
Figure 7. This figure represents the lower portion of housing. The upper portion was the replica of lower portion. The bearings are fitted in the holes shown in the figure.
Figure 8. This figure represents the central portion of housing. The gears are fitted in this portion. 8
M anufa anufacturing cturing Processes Involved Involved If we take few moments to inspect various objects around us (such as pencil, paper clip, table, light bulb, door knob, and cell phone), we shall soon realize that these objects have been transformed from various raw materials into individual parts and assembled into specific products. Some objects, such as nails, bolts, and paper clips, are made of one material; the vast majority of objects (such as toasters, bicycles, computers, washers and dryers, automobiles, and farm tractors) are, however, made of numerous parts from a wide variety of materials. A ballpoint pen, for example, consists of about a dozen parts, a lawn mower about 300 parts, a grand piano 12,000 parts, a typical automobile 15,000 parts, a C-5A transport plane more than 4 million parts, and a Boeing 747-400 about 6 million parts. All are produced by a combination of various processes called manufacturing.
Figu Fi gu r e 9. Classificati Classification on of Manufactu Manufacturing ring Processe Processess
The significant processes used in our project include Gear Hobbing, Wood Working, Sand Casting, Machining, Grinding and Press Fitting. 9
I. Gear H obbing There are different processes by which gears can be generated such as Form Milling, Gear Hobbing, Gear Shaping, Gear Broaching etc.; however we used the hobbing process. Gear hobbing is also a milling operation, but the cutter, called a hob, is much more complex and therefore much more expensive than a form milling cutter. In addition, special milling machines (called hobbing machines) are required to accomplish the relative speed and feed motions between the cutter and the gear blank. Gear hobbing is illustrated in Figure 10. As shown in the figure, the hob has a slight helix and its rotation must be coordinated with the much slower rotation of the gear blank in order for the hob’s cutting teeth to mesh with the blank’s teeth as they are being cut. This is accomplished for a spur gear by offsetting the axis of rotation of the hob by an amount equal to 90 less the helix angle relative to the axis of the gear blank. In addition to these rotary motions of the hob and the workpiece, a straight-line motion is also required to feed the hob relative to the gear blank throughout its thickness. Several teeth are cut simultaneously in hobbing, which allows for higher production rates than form milling. Accordingly, it is a widely used gear making process for medium and high production quantities.
Fig ur e 10. Gear Gear Ho Hobbin bbing g Proces Processs
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II. Sand Casting Sand casting is the most widely used casting process, accounting for a significant majority of the total tonnage cast. Nearly all casting alloys can be sand cast; indeed, it is one of the few processes that can be used for metals with high melting temperatures, such as steels, nickels, and titaniums. Its versatility permits the casting of parts ranging in size from small to very large and in production quantities from one to millions. Sand casting, also known as sand-mold casting, consists of pouring molten metal into a sand mold, allowing the metal to solidify, and then breaking up the mold to remove the casting. The casting must then be cleaned and inspected, and heat treatment is sometimes required to improve metallurgical properties. The cavity in the sand mold is formed by packing sand around a pattern (an approximate duplicate of the part to be cast), and then removing the pattern by separating the mold into two halves. The mold also contains the gating and riser system. In addition, if the casting is to have internal surfaces (e.g., hollow parts or parts with holes), a core must be included in the mold. Since the mold is sacrificed to remove the casting, a new sand mold must be made for each part that is produced. The patterns used in the casting process were made of wood.
Figure 11. Steps in the production sequence in sand casting. The steps include not only the casting operation but also pattern making and mold making.
III. Machining Machining is the most widely used manufacturing processes, because most of the industrial products have to be machined in order to bring them to net shape. For example, bar stock is the initial shape, but the final geometry after a series of machining operations is a shaft. Innumerable machining operations are performed in
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industries to achieve dimensional accuracies and other purposes, but we shall enlist few of those used in our project.
Facing: The tool is fed radially into the rotating work on one end to create a flat surface on the end.
Threading: A pointed tool is fed linearly across the outside surface of the rotating workpart in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder.
Boring: A single-point tool is fed linearly, parallel to the axis of rotation, on the inside diameter of an existing hole in the part.
Drilling: Drilling can be performed on a lathe by feeding the drill into the rotating work along its axis. Reaming can be performed in a similar way. These machining operations are illustrated in Figure given below.
Facing
Threading
Boring
Drilling Figure 12. Different Machining Operations
IV. G rindi rinding ng Grinding is a material removal process accomplished by abrasive particles that are contained in a bonded grinding wheel rotating at very high surface speeds. The grinding wheel is usually disk-shaped, and is precisely balanced for high rotational 12
speeds. Grinding can be likened to the milling process. Cutting occurs on either the periphery or the face of the grinding wheel, similar to peripheral and face milling. Peripheral grinding is much more common than face grinding. The rotating grinding wheel consists of many cutting teeth (the abrasive particles), and the work is fed relative to the wheel to accomplish material removal. Despite these similarities, there are significant differences between grinding and milling: (1) the abrasive grains in the wheel are much smaller and more numerous than the teeth on a milling cutter; (2) cutting speeds in grinding are much higher than in milling; (3) the abrasive grits in a grinding wheel are randomly oriented and possess on average a very high negative rake angle; and (4) a grinding wheel is self-sharpening— self-sharpening —as the wheel wears, the abrasive particles become dull and either fracture to create fresh cutting edges or are pulled out of the surface of the wheel to expose new grains.
Figu Fi gu re 13 . A Grinding Machine Machine with horizontal horizontal spindle spindle and reciproca reciprocating ting worktable. worktable.
V . Press Fitting Fitting A press fit assembly is one in which the two components have an interference fit between them. The typical case is where a pin (e.g., a straight cylindrical pin) of a certain diameter is pressed into a hole of a slightly smaller diameter. Standard pin sizes are commercially available to accomplish a variety of functions, such as (1) locating and locking the components— components—used to augment threaded fasteners by 13
holding two (or more) parts in fixed alignment with each other; (2) pivot points, to permit rotation of one component about the other; and (3) shear pins. Except for (3), the pins are normally hardened. Shear pins are made of softer metals so as to break under a sudden or severe shearing load to save the rest of the assembly. Other applications of press fitting include assembly of collars, gears, pulleys, and similar components onto shafts.
Figu Fi gu re 14 . Cross section of a solid solid pin or shaft assemble assembled d to a collar by interferenc interferencee fit.
Calculations Calcula tions for D is ischarge charge R ate of a G ear P um p The empirical formula for calculating discharge rate is given as Q = 0.95πC(D – C )lN Where Q = discharge rate in lpm (liters per minute) C = center to center distance between axis of gears (Center Distance) D = outside diameter of gears l = axial length of teeth (Face) N = speed in rpm Center D istance istance (C ) (in.)
Outside Diameter (D) (in.)
Face ( l) (in.)
Speed (N ) (rpm)
2.4
2.8
1.750
1500
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Conversions Center Distance = 2.4 in. = 0.6096 dm Outside Diameter = 2.8 in. = 0.7112 dm Face = 1.750 in. = 0.4445 dm Speed = 1500 rpm Q = 0.95πC(D 0.95πC(D – C )lN Q = (0.95)(3.14)(0.6096)(0.7112 – 0.6096)(0.4445)(1500) Q = 123.25 lpm
Ad vanta vantages ges and A pplic pplicati ations ons of an External G ear Pump The external gear pump is a positive displacement (PD) type of pump generally used for the transfer and metering of liquids. The pump is so named because it has two gears that are side-by-side or external to each other. (This nomenclature differentiates it from an internal gear pump, which has one gear positioned inside the other.)
The gear gear pump is a precision machine with extremely tight fits and and
tolerances, and is capable of working against high differential pressures. Another important advantage of the gear pump is its self-priming capability. Gear pumps are capable of self-priming because the rotating gears evacuate air in the suction line. This produces a partial vacuum that allows the atmospheric pressure to force the liquid into the inlet side of the pump. This ability of the gear pump makes it an ideal choice when the application requires that the pump be located above the liquid level, and the liquid must be lifted to the pump. Because a gear pump cannot create a perfect vacuum, the total lift (including pipe friction losses) should not exceed about 7.5 PSI, or about one-half of the atmospheric pressure. The tight clearances of the working parts inside a gear pump are what enable it to effectively pump liquids against high pressure. Low viscosity fluids such as alcohols and other solvents have more of a tendency to “slip” thru these tight spaces from the higher-pressure discharge side of the pump back to the lower-pressure suction side of the pump. The phenomenon of slip causes a reduction in flow rate and pump 15
efficiency. Slip depends on the magnitude of the differential pressure (i.e., the difference between the discharge and suction pressures), the viscosity of the liquid pumped and the working clearances inside the particular pump that is used. Slip increases with decreasing viscosity, increasing differential pressure and increasing gear-housing clearances, and is usually measured as a percent decrease from ideal flow (i.e., flow with zero slip). For fluid viscosities greater than about 50-100 cP (depending on the particular pump), the slip is minor, but it still depends on the differential pressure. This behavior is shown in Figure 15, which compares a typical gear pump’s performance curve for a thin fluid (such as water with a viscosity of about 1 cP at room temperature) with that of a moderately viscous fluid (such as a particular oil with a viscosity of 100 cP).
Figure 15. Performance curves for a typical external gear pump showing slip as a function of viscosity and differential pressure.
Gear pumps – properly designed and engineered – can offer many advantages. These include compactness, simplicity of design, easy serviceability, bi-directional flow capability, ability to self-prime, pulseless flow, low NPSHR (net positive suction head required), high MTBM (mean time between maintenance), high-pressure and high-temperature capability, precise and accurate metering, and availability in multiple seal configurations or sealless mag-drives. External gear pumps are used in 16
industrial and mobile (e.g. log splitters, lifts) hydraulic applications. Typical applications include:
Lubrication pumps in machine tools
Fluid power transfer units
Oil pumps in engines
Circulation
Injection
Drum transfer
Chemical additive and polymer metering
Chemical mixing and blending (double pump)
Acids and caustic (stainless steel or composite construction)
Conclusion In short, Gear Pumps are used in many industrial applications and manufacturing processes are a key element in the designing and functionality of gear pump. All we need to do is to apply these manufacturing processes effectively, so that we can upheave our industry.
References [1]. Frith, R. H., and Scott, W., 1996, ‘‘Comparison of an external gear pump wear model with test data,’’ Wear,196, pp. 64 –71. –71. [2]. Koc, E., and Hooke, C. C. J., 1997, ‘‘An experimental investigation into the design and performance of hydrostatically loaded floating wear plates in gear pumps,’’ Wear,209, pp. 184 –192. [3]. Koc, E., 1994, ‘‘Bearing misalignment effects effec ts on the hydrostatic and hydro dynamic behaviour behaviour of gears in fixed clearance end plates,’’ Wear,173, pp. 199 199––206. [4]. Koc, E., 1991, ‘‘An investigation into the performance of hydrostatically loaded end-plates end-plates in high pressure pumps and motors: Movable plate design,’’ Wear,141, pp. 249– 249 –265. 17
[5]. Foster, Foster, K., Taylor, R., and Bidhendi, I. M., 1985, ‘‘Computer prediction of cyclic excitation sources for an external gear pump,’’ Proc. Inst. Mech. Eng., Part C: Mech. Eng. Sci.,199, No. B3, pp. 175– 175–180. [6]. Chen, C. K., and Yang, S. C., 2000, ‘‘Geometric modeling for cylindrical and helical gear pumps with circular arc teeth,’’ Proc. Inst. Mech. Eng., Part C: J. Mech. Eng. Sci.,214, pp. 599– 599– 607. [7]. Mitome, K., and Seki, K., 1983, ‘‘A new continuous contact low-noise low -noise gear pump,’’ Journal of Mechanisms, Transmission and Automation in Design,105, pp. 736 –741. [8]. Manring, N. D., 2000, ‘‘The discharge flow ripple of an axial -piston swash-plate type hydrostatic pump,’’ ASME J. Dyn. Syst., Meas., Control,122, pp. 263 263––268. [9]. Kalpakjian S. and Schmid, S.R. (2008). Manufacturing Processes for Engineering Materials . (5th ed., p. 1). NJ , USA: Pearson Education, Inc. [10]. Groover M.P. (2010). Fundamentals of Modern Manufacturing (Materials, Processes and Systems). Systems). (4th ed., pp. 225-227, 510-513, 604-605, 618, 541-542). USA: John Wiley and Sons, Inc. [11]. Yadav, S. K. (2010). To design, fabrication and testing of gear pump test rig. Project Report, Galgotias College of Engineering and Technology, Mechanical Engineering. nd
[12]. Gear Pump, Bearings, Gears, Drive Shaft. (May 22 , 2012 .). Retrieved from Wikipedia: http://www.wikipedia.com.
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